South Korea and the United States have launched a joint naval exercise off the east coast of the Korean peninsula to test the combat readiness of the two allies.

The three-day drill that began Monday comes at a time North Korea is ramping up daily threats of a nuclear test in response to expand U.N. sanctions imposed after its long-range rocket launch in December.

The joint U.S. - South Korean drills are taking place in the East Sea near the city of Pohang. The exercise includes naval maneuvers, submarine detection, live-fire drills and antiballistic missile drills.

The South Korean navy deployed 10 vessels, while the U.S. Navy has the nuclear submarine USS San Francisco and the guided missile cruiser USS Shiloh in the maneuvers.

In talks by telephone Sunday, U.S. Secretary of State John Kerry and South Korean Foreign Minister Kim Sung-nwan agreed that a third nuclear test by North Korea should be met by a "firm response."